Magnetic recording medium

ABSTRACT

A magnetic recording medium has on a nonmagnetic support, a magnetic layer consisting mainly of an acicular magnetic powder and a binder. The medium has excellent magnetic characteristics required for short-wave recording and has an excellent durability. For this purpose, the medium contains a ferromagnetic metal powder which has a coercive force of not less than 1,000 Oe and a specific surface area as determined by the BET method, of not less that 45 m 2  /g. The medium contains in the binder not less than 50% by weight of a resin which has a high dispersibility owing to the introduction of at least one polar group selected from the group consisting of polar groups having the general formulas of: ##STR1## (wherein M is hydrogen, lithium, sodium or potassium; and M 1  and M 2  are hydrogen, lithium, sodium, potassium or alky groups).

TECHNICAL FIELD

The present invention relates to a magnetic recording medium and, moreparticularly, to a short-wave recording magnetic recording medium inwhich a magnetic layer having a high coercive force and an excellentdispersibility is formed on a nonmagnetic support.

BACKGROUND ART

A tendency toward short-wave recording is significant for high-densityrecording in magnetic recording media, especially, magnetic recordingmedia for VTRs. In order to obtain a high reproducing output inshort-wave recording, it is indispensable that a magnetic layer of amagnetic recording medium has a high coercive force and a high residualflux density. However, such satisfactory magnetic characteristics cannotbe obtained in a current magnetic recording medium which uses as amagnetic powder iron oxide magnetic powders such as γ--Fe₂ O₃ or achromium dioxide magnetic powder. The central recording wavelengthattainable in practice is limited to about 1 μm.

In contrast to this, in a magnetic recording medium using aferromagnetic powder as a magnetic powder, a coercive force Hc and aresidual flux density Br of a magnetic layer formed thereon are bothhigh, so that high reproducing output can also be obtained in ashort-wave band. Since such a medium permits high-density recording, itis considered to be most promising and various associated developmentsare being made.

In Japanese Patent Application Nos. 56-20388 and 56-40202, the presentapplicant has previously proposed a magnetic recording medium which usessuch a ferromagnetic metal powder and which is therefore suitable forshort-wave recording, especially, short-wave recording having a centralrecording wavelength of 1 μm or less. However, in such a magneticrecording medium, when a fine ferromagnetic metal powder having a greatspecific surface area as used to improve, for example, the S/N ratio,various problems such is degradation in dispersibility of theferromagnetic metal powder in a binder and an increase in the viscosityof a magnetic coating are raised. These problems adversely affect thesurface properties, durability, magnetic characteristics and so on of amagnetic layer, which prevents the manufacture of a high-performancemagnetic recording medium. This also impairs the workability in themanufacture of a magnetic recording medium. In order to improve thedispersibility of the magnetic powder, methods have been proposedwherein a surfactant is used as a dispersant, and the content of ahydrophillic group such as an OH group in the binder is increased.However, in either case, a satisfactory dispersibility can not beobtained. Moreover, it has been found that physical properties andmechanical properties and abrasion resistance, such as especiallyrubbing-off, of a magnetic layer are largely degraded.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a magnetic recordingmedium for short-wave recording which uses a ferromagnetic metal powderas a magnetic powder, which has an improved dispersibility of themagnetic powder, a low noise level, and excellent magneticcharacteristics such as squareness ratio.

For this reason, the present invention relates to a magnetic recordingmedium having, on a nonmagnetic support, a magnetic layer consistingmainly of an acicular ferromagnetic metal powder and a binder,characterized in that the metal powder has a coercive force of not lessthan 1,000 Oe and a specific surface area determined by the BET methodis not less than 45 m^(2/) g, said binder containing not less than 50%by weight of a resin having at least one polar group selected from thegroup consisting of polar groups having the general formulas: ##STR2##(wherein M is hydrogen, lithium, sodium, or potassium; and M₁ and M₂ arehydrogen, lithium, sodium, potassium or alkyl groups).

According to the present invention, since a fine ferromagnetic metalpowder having a high coercive force is used in combination with a binderhaving a polar group, a short-wave recording magnetic recording mediumis obtained which has excellent magnetic characteristics and a gooddispersibility of the ferromagnetic metal powder, resulting in excellentsurface properties, durability and performance.

BEST MODE OF CARRYING OUT THE INVENTION

In a magnetic recording medium in accordance with the present invention,the coercive force Hc of a magnetic powder is selected to be not lessthan 1,000, preferably 1,000 to 2,000 Oe, and more preferably 1,100 to1,500 Oe. When short-wave recording is to be performed, the coerciveforce Hc must be considerably high, that is, preferably not less than1,000 Oe. However, when the coercive force is excessively high, asaturation occurs in a magnetic head during recording, and erasure ofrecorded contents becomes difficult.

In order to obtain a coercive force Hc of not less than 1,000 Oe, sincethe coercive force Hc depends on the shape anisotropy of a ferromagneticmetal particle, the axial ratio of the ferromagnetic metal particle,that is, the ratio of the major axis to the minor axis, is selected tobe not less than 7 and preferably not less than 10.

The thickness after drying of a coating layer as a magnetic layer ispreferably 0.5 to 6 μm. When the coated thickness is too small, auniform coating layer is difficult to form, which results in a signalloss, i.e., a so-called drop-out. On the other hand, when the coatedthickness is too large, thickness loss due to self-demagnetizationoccurs.

A weight ratio P/B of the magnetic powder and the binder constituting amagnetic layer is selected to be 5 to 12 and preferably 6 to 10 for eachpart of binder. When the amount of the binder is too large, that is,when the ratio P/B is too small, a residual flux density Br, is toosmall, and an improvement in the S/N ratio cannot be expected. On theother hand, when the ratio P/B is too large, the rubbing-off isincreased, and the durability is degraded.

In accordance with the present invention, a ferromagnetic metal powderfor a magnetic layer having a specific surface area as determined by theBET method, of not less than 45 m^(2/) g is used. This is attributed tothe following reasons. With particles having a specific surface area ofless than 45 m^(2/) g are used, a satisfactory effect of improving theS/N ratio cannot be expected. On the other hand, if the specific surfacearea exceeds 45 m^(2/) g, when modulation noise is measured, a desiredlow noise level is obtained even in the case of short waves having arecording wavelength of 1 μm or less. The specific surface area may beincreased above 45 m^(2/) g or to a degree that the powder is notrendered ultra-paramagnetic. However, in practice, the specific surfacearea preferably has a range of 45 to 150 m^(2/) g.

A ferromagnetic metal powder to be used herein may be an acicular metalpowder of Fe, Fe--Co, Fe--Co--Ni, or the like or, when consideringcorrosion resistance and prevention of the sintering during manufacture,may be an acicular metal powder containing a small amount of an additiveelement such as Al, Cr, or Si. These acicular metal powders can beprepared by reducing, in a reducing atmosphere such as H₂ gas, anacicular iron oxide, water-containing iron oxide, or iron oxide orwater-containing iron oxide optionally containing a metal of such as Niand Co, as a starting material. The specific surface area of theferromagnetic metal powder can be controlled by properly selecting thespecific surface area of the aforementioned starting material.

In a magnetic recroding medium of the present invention, the bindercontains a resin having at least one polar group selected from the groupconsisting of polar groups having the general formulas: ##STR3##(wherein M is a hydrogen, lithium, sodium, or potassium; and M₁ and M₂are hydrogen, lithium, sodium, potassium or alkyl groups).

In the general formulas above, the alkyl groups are preferablysaturated, straight or branched chain aliphatic hydrocarbon residueshaving up to 23 carbon atoms. Examples of such an alkyl group mayinclude a tricosyl group, docosyl group, eicosyl group, nonadecyl group,octadecyl group, hexadecyl group, tridecyl group, undecyl group, decylgroup, nonyl group, octyl group, hexyl group, pentyl group, butyl group,t-butyl group, propyl group, isopropyl group, ethyl group, and methylgroup. Note that M₁ and M₂ may be the same or different. Since such apolar group has a large polarity and has a good compatibility with aferromagnetic metal powder, it improves the dispersibility of theferromagnetic metal.

In accordance with the present invention, the resin containing theaforementioned polar group is contained in the binder in an amount ofnot less than 50% by weight based on the total binder components. Whenthe amount of the resin is less than 50% by weight, a satisfactrydispersibility cannot be obtained, so that the improvements in thesurface properties and the S/N ratio cannot be expected. When adispersant is used to compensate this, the rubbing-off is increasedsignificantly to an impractical degree.

A resin having a polar group to be used herein has preferably amolecular weight of not more than 50,000. If the molecular weight of theresin exceeds 50,000, when the specific surface area determined by theBET method, of a magnetic powder as shown in FIG. 1 becomes large, thecoating viscosity becomes too high, exceeding an acceptable value. Theresin preferably has a molecular weight of not less than 2,000. Thereason is that, if the molecular weight of the resin is less than 2,000,when the magnetic coating is applied on a support and is hardened with ahardener, an unreacted portion is formed and low molecular weightconstituents remain, which will degrade the physical properties of thecoating layer.

The resin having the polar group used herein preferably has a molecularweight of 200 to 50,000 per polar group. When this molecular weight islower than 200 the hydrophillic nature of the resin is too strong sothat the solubility in a solvent, compatibility with other resins in thebinder, and humidity resistance of a magnetic layer are degraded. On theother hand, when this molecular weight exceeds 50,000, an effect ofimproving the dispersibility is unsatisfactory.

A resin having the aforementioned polar group according to the presentinvention may be a polyester resin, a polyurethane resin, a vinylchloride-based resin, into which the polar group has been introduced.Such a resin can be prepared by various methods. For example, apolyester resin containing a metal sulfonate group can be prepared byusing a dicarboxylic acid containing a metal sulfonate group as part ofa dicarboxylic acid component to be used, and condensing thedicarboxylic acid containing the metal sulfonate group and adicarboxylic acid having no metal sulfonate group with a diol. Apolyurethane resin containing a metal sulfonate group can be prepared byusing three types of starting materials of polyester containing themetal sulfonate group together with a diisocyanate so as to perform acondensation reaction and an addition reaction. There may be furtheradopted a method wherein a polar group is introduced into a polyesterresin, a polyurethane resin or a vinyl chloride-based resin bymodification. According to this method, the polar group can beintroduced into the resins by conducting condensation throughdehydrochlorination reaction of the resins with a compound containing inmolecules the polar group and chlorine, such as C1--CH₂ CH₂ SO₃ M,C1--CH₂ CH₂ OSO₃ M, C1--CH₂ COOM, and ##STR4##

Carboxylic acid components to be used herein to prepare polyester resinsor polyurethane resins may be an aromatic dicarboxylic acids such asterephthalic acid, isophthalic acid, orthophthalic acid, or1,5-naphthalic acid; aromatic hydroxycarboxylic acids such asp-hydroxybenzoic acid or p-(hydroxy-ethoxy) benzoic acid; aliphaticdicarboxylic acids such as succinic acid, adipic acid, azelaic acid,sebacic acid, or dodecane dicarboxylic acid; or tri- or tetracarboxylicacids such as trimellitic acid, trimesic acid, or pyromellitic acid.Terephthalic acid, isophthalic acid, adipic acid, and sebacic acid areparticularly preferable.

Dicarboxylic acid components containing metal sulfonate groups may, forexample, be 5-sodium sulfoisophthalate, 5-potassium sulfoisophthalate,2-sodium sulfoterephthalate, or 2-potassium sulfoterephthalate.

Diol components may, for example, be ethylene glycol, propylene glycol,1, 3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,neopentyl glycol, diethylene glycol, dipropylene glycol,2,2,4-trimethyl-1,3-pentanediol, 1,4-cyclohexanedimethanol, an ethyleneoxide adduct and propylene oxide adduct of bisphenol A, an ethyleneoxide adduct of hydrogenated bisphenol A, polyethylene glycol,polypropylene glycol, polytetramethylene glycol, or the like.Additionally, tri- or tetraols such as trimethylol ethane, trimethylolpropane, glycerin, or pentaerythritol may be used.

Diisocyanate components which may be used herein in order to preparepolyurethane resins may be 2, 4-tolylenediiscyanate,2,6-tolylenediisocyanate, p-phenylenediisocyanate,diphenylmethanediisocyanate, m-phenylenediisocyanate,hexamethylenediisocyanate, tetramethylenediisocyanate,3,3'-dimethoxy-4,4'-biphenylenediisocyanate,2,4-naphthalenediisocyanate, 3,3'-dimethyl-4,4'-biphenylenediisocyanate,4,4'-diisocyanate-diphenyl ether, 1,3-naphthalenediisocyanate,p-xylylenediisocyanate, m-xylylenediisocyanate,1,3-diisocyanatemethylcyclohexane, 1,4-diisocyanatemethylcyclohexane,4,4'-diisocyanatedicyclohexane, 4,4'-diisocyanatedicyclohexylmethane,isophoronediisocyanate, or the like.

When a metal sulfonate group is introduced by modifying a vinylchloride-based resin, the vinyl chloride-based resin may be vinylchloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinylpropionate-vinyl alcohol copolymer, vinyl chloride-vinylpropionate-vinyl aclcohol copolymer, vinyl chloride-vinyl acetate-vinylmaleate-vinyl alcohol copolymer, vinyl chloride-vinyl propionate-vinylmaleate-vinyl alcohol copolymer or the like. The OH group of the vinylalcohol contained in such a vinyl chloride-based resin is subjected to adehydrochlorination reaction with chlorine in a metal sulfonatecontaining chlorine such as C1--CH₂ CH₂ SO₃ M or C1--SO₃ M in a polarorganic solvent such as dimethylformamide or dimethyl sulfoxide in thepresence of dehydrochlorinating agents such as amines, e.g., pyridine,picoline or triethylamine, or epoxy compounds such as ethylene oxide orpropylene oxide.

To a magnetic layer may further be added aluminum oxide, chromium oxide,silicon oxide or the like as a reinforcing agent; olive oil or the likeas a lubricant; carbon black or the like as a antistatic agent; orlecithin or the like as a dispersant.

The magnetic layer is generally formed by dissolving components of themagnetic layer in an organic solvent to prepare a magnetic coating, andthen by applying the magnetic coating on a nonmagnetic support. Suitableorganic solvents to be used herein may be ketones (e.g., acetone, methylethyl ketone, methyl isobutyl ketone, or cylcohexanone), alcohls (e.g.,methanol, ethanol, propanol, or butanol), esters (e.g., methyl acetate,ethl acetate, butyl acetate, ethyl lactate, or glycol acetate),monoethyl ether, glycol ethers (e.g., ethylene glycol dimethyl ether,ethylene glycol monoethyl ether, or dioxane), aromatic hydrocarbons(e.g., benzene, toluene, or xylene), aliphatic hydrocarbon (e.g., hexaneor heptane), nitropropane or the like.

The support on which a magnetic coating is applied is nonmagnetic andmay be materials such as a polyester (e.g., polyethylene terephthalate),polyolefins, (e.g., a polypropylene), cellulose derivatives (e.g.,cellulose triacetate or cellulose diacetate), polycarbonate, polyvinylchloride, polyimide, polyamide, polyhydrazide, metals (e.g., aluminum orcopper), paper or the like.

The present invention will now be described by way of examples.

EXAMPLE 1

    ______________________________________                                        Ferromagnetic metal powder                                                                         670 parts by weight                                      (specific surface area:                                                       45 m.sup.2 /g; axial ratio: 11-13)                                            Polyurethane resin containing                                                                      100 parts by weight                                      a sodium sulfonate group                                                      (molecular weight: 18,000)                                                    Cr.sub.2 O.sub.3 particles                                                                          60 parts by weight                                      Carbon black          35 parts by weight                                      Olive oil             7 parts by weight                                       Toluene              350 parts by weight                                      Methyl ethyl ketone  350 parts by weight                                      Cyclohexanone        350 parts by weight                                      ______________________________________                                    

The above composition was milled and dispersed in a ball mill for 20hours. Then, after adding 28 parts of an isocyanate compound ("DesmodurL": manufactured by Bayer Co., Ltd.), the mixture was dispersed by highspeed shearing to prepare a magnetic coating.

The magnetic coating was applied on one surface of a polyethyleneterephthalate base having a thickness of 14 μm and a surface roughnessof 0.03 μm such that the coating layer will have a dried thickness of4.0 μm. The layer was oriented in a D.C. magnetic filed of 2,500 Gauss,dried at 80° C., and subjected to supercalendering. The base with thelayer was cut into a width of 1/2 inch to prepare a magnetic tape. Themagnetic characteristics of the magnetic tape were measured. The resultsobtained are given in Table 3.

EXAMPLES 2-5

Magnetic tapes were prepared following the procedures Example 1 exceptthat a polyurethane resin containing a sodium sulfonate group and avinyl chloride-vinyl acetate-vinyl alcohol copolymer (VAGH: manufacturedby U.C.C.) were used in the amounts designated in Table 1 in place of100 parts by weight of the polyurethane resin containing a sodiumsulfonate group. The magnetic characteristics of the resultant tapeswere measured. The results obtained are shown in Table 3.

                  TABLE 1                                                         ______________________________________                                                  Added amount of poly-                                                                         Added amount of                                               urethane resin con-                                                                           vinyl chloride-                                               taining sodium sulfonate                                                                      vinyl acetate-                                      Examples  group           copolymer                                           ______________________________________                                        1         100 parts by weight                                                                           --                                                  2         80 parts by weight                                                                            20 parts by weight                                  3         50 parts by weight                                                                            50 parts by weight                                  4         40 parts by weight                                                                            60 parts by weight                                  5         10 parts by weight                                                                            90 parts by weight                                  ______________________________________                                    

EXAMPLE 6

    ______________________________________                                        Ferromagnetic metal powder                                                                         670 parts by weight                                      (specific surface area:                                                       60 m.sup.2 /g; axial ratio: 11 to 13)                                         Polyester resin containing                                                                         100 parts by weight                                      sodium sulfonate group                                                        (molecular weight: 20,000)                                                    Cr.sub.2 O.sub.3 particles                                                                          60 parts by weight                                      Carbon black          35 parts by weight                                      Olive oil             7 parts by weight                                       Toluene              350 parts by weight                                      Methyl ethyl ketone  350 parts by weight                                      Cyclohexanone        350 parts by weight                                      ______________________________________                                    

A magnetic tape was prepared using the above composition and followingthe procedures similar to Example 1. The results obtained are given inTable 3.

EXAMPLES 7-9

Magnetic tapes were prepared in a similar manner to Example 6 excepttnat a polyester resin containing a sodium sulfonate group and athermoplastic polyurethane resin ("N-3022": manufactured by NipponPolyurethane Kogyo K.K.) were used in the amounts designated in Table 2in place of 100 parts by weight of the polyester resin containing asodium sulfonate group. The magnetic characteristics of the resultantmagnetic tapes were measured. The results obtained are given in Table 3.

                  TABLE 2                                                         ______________________________________                                                 Added amount of poly-                                                                          Added amount of                                              ester resin containing                                                                         thermoplastic poly-                                 Examples sodium sulfonate group                                                                         urethane resin                                      ______________________________________                                        6        100 parts by weight                                                                            --                                                  7        70 parts by weight                                                                             30 parts by weight                                  8        50 parts by weight                                                                             50 parts by weight                                  9        40 parts by weight                                                                             60 parts by weight                                  ______________________________________                                    

EXAMPLE b 10

    ______________________________________                                        Ferromagnetic metal powder                                                                           670 parts by weight                                    (specific surface area:                                                       45 m.sup.2 /g; axial ratio: 11 to 13)                                         Vinyl chloride-Vinyl Acetate-                                                                         80 parts by weight                                    vinyl alcohol copolymer con-                                                  taining sodium sulfonate group                                                Thermoplastic polyurethane resin                                                                      20 parts by weight                                    ("N-3022": manufactured by Nippon                                             Polyurethane Kogyo K.K.)                                                      Cr.sub.2 O.sub.3 particles                                                                            60 parts by weight                                    Carbon black            35 parts by weight                                    Olive oil               7 parts by weight                                     Toluene                350 parts by weight                                    Methyl ethyl ketone    350 parts by weight                                    Cyclohexanone          350 parts by weight                                    ______________________________________                                    

A magnetic tape was prepared using the above composition in the mannersimilar to Example 1. The magnetic characteristics of the tape weremeasured. The results obtained are given in Table 3.

                  TABLE 3                                                         ______________________________________                                                Coercive Residual  Square-                                                    force    flux den- ness   RF out- C/N                                 Examples                                                                              Hc(Oe)   sity Br(G)                                                                              ratio  put (dB)                                                                              (dB)                                ______________________________________                                        1       1,190    2,810     0.79   0       50.9                                2       1,190    2,760     0.78   -0.2    50.7                                3       1,195    2,575     0.75   -0.6    50.2                                4       1,200    2,210     0.69   -1.4    48.8                                5       1,210    1,820     0.66   -2.5    47.1                                6       1,230    2,710     0.78   -0.1    54.3                                7       1,230    2,640     0.76   -0.4    53.9                                8       1,235    2,390     0.71   -0.7    53.5                                9       1,245    1,870     0.68   -2.4    50.8                                10      1,190    2,760     0.78   -0.1    50.6                                ______________________________________                                    

As can be seen from Table 3, the magnetic tapes in which the bindercontains the resin containing a metal sulfonate group, in the amount ofnot less than 50% (Examples 1 to 3, 6 to 8, and 10) has maintained theC/N ratio of 50 dB or more. Good results were also obtained with respectto the residual flux density, the squareness ratio, and high-frequency(RF) output. This is due to the improvement in the surface properties ofthe magnetic layer by improved dispersibility of the magnetic powder.The improvement in the surface properties also resulted in animprovement in durability.

Since the binder containing a polar group according to the presentinvention has itself a high dispersibility, it eliminates the use of anadditional dispersant or requires only a small amount of such anadditional dispersant. In correspondence with such a reduction in theamount of dispersant to be used, the amount of the nonmagnetic materialfor improving durability can be increased, so that the durabilitycharacteristics such as rubbing-off characteristics and wear resistanceare improved.

We claim:
 1. A magnetic recording medium having on a nonmagnetic supporta magnetic layer measuring from 0.5 to 6 microns and consisting mainlyof an acicular ferromagnetic metal powder and a binder, characterized inthat said acicular ferromagnetic metal powder has a coercive force ofnot less than 1,000 Oe, a specific surface area as determined by the BETmethod, of not less than 45 m² /g, an axial ratio of not less than 7,and said binder contains not less than 50% by weight of a resin having amolecular weight not in excess of 50,000 and containing at least onepolar group selected from the group consisting of polar groups havingthe general formulas: ##STR5## (wherein M is hydrogen, lithium, sodiumor potassium; and M₁ and M₂ are hydrogen, lithium, sodium, potassium oralkyl groups), the weight ratio of feromagnetic metal powder to binderbeing in the range of 5 to
 12. 2. A magnetic recording medium accordingto claim 1, characterized in that said acicular metal powder has acoercive force of 1,000 to 2,000 Oe.
 3. A magnetic recording mediumaccording to claim 1, characterized in that said acicular ferromagneticmetal powder has the coercive force of 1,100 to 1,500 Oe.
 4. A magneticrecording medium according to claim 1, characterized in that an axialratio of said acicular ferromagnetic metal powder is not less than 10.5. A magnetic recording medium according to claim 1, characterized inthat said alkyl group is a saturated, straight or branched chainaliphatic hydrocarbon group having up to 23 carbon atoms.
 6. A magneticrecording medium according to claim 1, characterized in that themolecular weight of said resin having the polar group is 2,000 to50,000.
 7. A magnetic recording medium according to claim 1,characterized in that said resin having the polar group has a molecularweight of 200 to 50,000 per polar group.
 8. A magnetic recording mediumaccording to claim 1, characterized in that a main chain of said resinhaving the polar group is polyester, polyurethane or vinylchloride-based copolymer.